Diode lasers are formed from a long semiconductor bar fabricated with a row of spaced apart emitters. A typical diode bar is a 10 mm long semiconductor bar fabricated with 19 to 79 emitters having widths of 100 to 200 μm. Fill factors can be from 8% to 80% giving variable pitch to fill the length. To increase the power output, the diode bars are stacked with a bar pitch in the range 1 to 2 mm for CW stacks, while for QCW stacks pitch can be smaller, in the range 300 μm to 2 mm. Commercially produced units, emitting 50-100 W per bar, are stacked on typically 1.8 mm pitch to build up a total laser power of 500-1000 W. However, power levels up to 500 W per bar on QCW systems, are achievable.
While each emitter is designed to be identical, manufacturing tolerances and environmental influences mean that each emitter produces a beam of variable wavelength, spectral shape, beam angle and divergence. Consequently these individual beams must be combined effectively to provide a suitable output in terms of power, brightness and beam quality for a desired application.
An increasing application area is in laser material processing. High brightness is required in order to deliver sufficient power with a specified spot size at the workpiece. Also, it is often desirable to deliver the output of the diode laser to the workpiece via an optical fibre. High brightness is then required in order to launch sufficient power into the limited etendue of the optical fibre. Wavelength division multiplexing provides a means of combining the beams from multiple diode laser sources to achieve increased brightness. Wavelength division multiplexing of a large number of sources requires dense spectral combining. Many practical schemes for dense spectral combining require sources of narrow linewidth at uniformly-spaced wavelength intervals.
It is known that use of a volume holographic grating or volume Bragg grating can stabilize and lock the wavelength of a single emitter. A single volume holographic grating has also been used on a laser diode bar to lock all the emitters to a single wavelength over a narrow spectrum. With the addition of collimating optics this provides an array of output beams matched to the centre frequency of the grating. The system can be scaled to stacks of diode bars by either using multiple volume holographic gratings or a single volume holographic grating with suitable dimensions. These scaled arrangements provide power scaling without increasing brightness as the output is centred at a single wavelength with a narrow spectrum.
Combining optical beams of multiple wavelengths provides increased brightness. For each emitter to be locked to a unique wavelength would require a volume holographic grating with a different resonant wavelength for each emitter in an array. This could be achieved using a large number of gratings at stepped increments in wavelength. This would require a large number of well-toleranced grating parts, and would be complex to assemble. This has been overcome by developing a wavelength chirped volume holographic grating. A typical wavelength chirped volume holographic grating is manufactured to have dimensions comparable to the emitter array, with the wavelength chirped in the bar axis, the stack axis or both.
Typically a wavelength chirped volume holographic grating is mounted directly in front of the emitter array. Collimating optics may be located between the emitters and the wavelength chirped volume holographic grating to reduce divergence, avoid overlapping of the beams and ensure that they are incident at right angles to the wavelength chirped volume holographic grating. In some applications, the grating is wavelength chirped on a single axis. In this way, all the emitters on a laser diode bar are locked to the same wavelength, with each bar operating on a different wavelength. The output is then combined optically to provide a multi-wavelength beam.
A disadvantage in the use of wavelength chirped volume holographic gratings is in the cost of their manufacture. These are expensive components as compared to uniform volume holographic gratings which are tuned to a single frequency. Adoption of one or more wavelength chirped volume holographic gratings in a laser arrangement significantly increases the manufacturing costs to provide the high spectral brightness desired from such lasers.
It is therefore an object of the present invention to provide a multi-wavelength laser array without the requirement of a wavelength chirped volume holographic grating.
It is a further object of the present invention to provide a multi-wavelength laser array using a uniform volume holographic grating.
It is a further object of at least one embodiment of the present invention to provide a multi-wavelength laser array from a diode laser bar.